1. Field of the Invention
The present invention relates to a laser beam irradiation method and a laser irradiation apparatus for using the method (apparatus including a laser and an optical system for guiding laser beam emitted from the laser to an object to be illuminated). In addition, the present invention relates to a method of manufacturing a semiconductor device, which includes a laser beam irradiation step. Note that a semiconductor device described here includes an electro-optical device such as a liquid crystal display device or a light-emitting device, and an electronic device that includes the electro-optical device as a part.
2. Description of the Related Art
In recent years, an extensive study has been made on a technique in which an amorphous semiconductor film formed on an insulating substrate made of glass or the like is crystallized so that a semiconductor film having crystal structure (hereafter referred to as crystalline semiconductor film) is obtained. As the methods of crystallization such as a thermal annealing method using furnace annealing, a rapid thermal annealing method (RTA method), a laser annealing method and the like were examined. Anyone thereof or combining two or more methods thereof can be carried out for crystallization.
In comparison with an amorphous semiconductor, a crystalline semiconductor film has extreme high mobility. Since thus, the crystalline semiconductor film is used to form a thin film transistor (referred to as TFT), for example, the TFT can be widely used in an active matrix liquid crystal display device in which TFTs for pixel portion, TFTs for pixel portion and TFTs for driver circuit are formed on one glass substrate.
Generally, in order to crystallize an amorphous semiconductor in annealing furnace, a thermal treatment at 600xc2x0 C. or more for 10 hours or more is required. A quartz is an applicable material of substrate for this crystallization, but the quartz substrate is too expensive in price to be manufactured especially in a large area. In order to improve the productivity efficiency, manufacturing the substrate in a large area is unavoidable, it is expected that a substrate in which a length of one side exceeds 1 m will be also used in recent years.
On the other hand, a method of thermal crystallization by using metal elements disclosed in Japanese Patent Application Laid Open No. 7-183540 enable the crystallization temperature which was a conventional problem to be realized at a low temperature. The crystalline semiconductor film can be formed by this method in which a small amount of an element such as nickel, palladium and lead is added to an amorphous semiconductor film, then the amorphous semiconductor film is heated. for four hours at 550xc2x0 C.
Since the laser annealing method can deliver high energy only to the semiconductor film without substantially increasing the temperature in substrate, the laser annealing technology comes under spotlight by its appliance in a glass substrate with a low strain point as a matter of course, and a plastic substrate, etc.
An example of the laser annealing method is a method of forming pulse laser beam from an excimer laser or the like by an optical system such that it becomes a square spot of several cm or a linear shape of 100 mm or more in length on a surface being illuminated, and relatively shifting an irradiation position of the laser beam with respect to the surface being illuminated to conduct annealing. The xe2x80x9clinear shapexe2x80x9d described here means not a xe2x80x9clinexe2x80x9d in the strict sense but a rectangle (or a prolate ellipsoid shape) having a high aspect ratio. For example, although, it indicates a shape having an aspect ratio of 2 or more (preferably, 10 to 100), it doesn""t make any difference from that a shape at a surface being illuminated is being contained in the laser light having rectangular shape (rectangular shape beam). Note that the linear shape is used to obtain an energy density required for annealing an object sufficiently to be illuminated. Thus, if sufficient annealing is conducted for the object to be illuminated, it may be a rectangular shape and a tabletop shape.
However, a crystalline semiconductor film formed by subjecting an amorphous semiconductor film to laser annealing includes a collection of a plurality of crystal grains, and the position and size of the crystal grains are random. TFTs are formed on a glass substrate by patterning the crystalline semiconductor layer in an island shape for device separation. In this case, the position and size of crystal grains cannot be specified. In comparison with the inner of crystal grains, the interface of crystal grains has an infinite number of a recombination centers or a trapping centers caused by an amorphous structure, a crystal defect, and the like. If the carriers are trapped in trapping centers, potential at a grain boundary will be increased and become barriers to carriers, it is known that current transporting characteristics of carriers will be degraded caused by this. However, it is almost impossible to form a channel formation region by using a single crystal semiconductor film while avoiding the influence of a crystal boundary, although crystal characteristics of semiconductor film of channel formation region have a serious effect on the TFT characteristics.
There is a crystal growth technology that is recently attracting attention. In the technology, when a CW laser is illuminated on a semiconductor film with the CW laser scanning in one direction, crystal grains grow connected in the scanning direction thereof, resulting in forming a single crystal elongated in that direction. It is considered that when this method is applied, a semiconductor film that has no grain boundary at least in a channel direction of a TFT can be formed. However, in this method, since a CW laser having a wavelength in a region that can be sufficiently absorbed by the semiconductor film is used, only a laser that is very small in its output such as substantially 10 W can be applied. Accordingly, in view of productivity, it is inferior to technology that uses an excimer laser.
The present invention intends to provide a method for, with a CW laser, illuminating a laser light with high production efficiency and a laser irradiation apparatus for carrying out the irradiation of the laser light. In addition, the present invention also intends to provide a method for fabricating a semiconductor device by use of a semiconductor film obtained by carrying out the laser irradiation like this.
In a process of crystallizing a semiconductor film with a CW laser, in order to improve the productivity even a little, the following is actively carried out. That is, a laser beam is processed into a long ellipse in a surface being illuminated and the processed laser beam is scanned in a minor axis direction of the elliptical laser beam (hereinafter referred to as an elliptical beam), and thereby the semiconductor film is crystallized. The present invention intends to provide a method for illuminating an elliptical beam with the highest productivity in the process like this.
The CW laser suitable for the present method is one that has an wavelength in the range of 550 nm or less and a remarkably high output stability, for instance, second harmonics of a YVO4 laser, second harmonics of a YAG (Nd3+: YAG, Cr4+: YAG) laser, second harmonics of a YLF laser, second harmonics of a glass laser, second harmonics of a YalO3 laser, second harmonics of a Y2O3 (Nd3+:Y2O3, Yb3+:Y2O3) laser, and Ar laser being applicable. Alternatively, further higher order harmonics of the above lasers may be used. Further alternatively, lasers such as a ruby laser, an alexandrite laser, a Ti: sapphire laser, a CW excimer laser, an Ar laser, a Kr laser, a CO2 laser, a CW helium-cadmium laser, a copper vapor laser, and a gold vapor laser may be used. A plurality of one kind of these lasers or a plurality of kinds of these lasers may be used.
First, a 10 W YVO4 laser (CW, the second harmonics, TEM00) is prepared and a beam shape thereof is processed with a convex lens having a focal length of 20 mm into an elliptical beam. Specifically, a laser beam is obliquely entered in the convex lens and processed into a slender elliptical beam by use of astigmatism or the like. The present experiment will be explained with reference to FIG. 2. In the present experiment, a laser beam exited from a laser oscillator 201 is reflected by a mirror 202, and entered into a convex lens 203 at an angle of incidence of 20 degree, and an elliptic beam 205 having a major axis of substantially 500 xcexcm and a minor axis of substantially 30 xcexcm is formed on a semiconductor film 204 disposed on an surface being illuminated in parallel with the convex lens 203. Though an irradiation efficiency can be improved by further shortening the minor axis and thereby lengthening the major axis, since as a length of the minor axis becomes shorter a focal depth becomes shallower, and a uniform laser annealing becomes difficult to perform, the above sizes are considered appropriate.
When the semiconductor film 204 is scanned in a direction of the minor axis of the elliptical beam 205, in a region having a width of 150 xcexcm in a major axis direction of the elliptical beam, grains extended in a scanning direction are formed closely packed. Hereinafter, the region is referred to as a width of a long grain region. The semiconductor film is formed on a glass substrate. Specifically, on one surface of a glass substrate having a thickness of 0.7 mm, silicon oxynitride is deposited with a thickness of 200 nm, and thereon an a-Si film having a thickness of 150 nm is deposited by use of the plasma CVD method. Furthermore, in order to improve resistance properties of the semiconductor film against the laser, the semiconductor film is exposed to thermal annealing at 500 degree centigrade for 1 hr. Other than the thermal annealing, as mentioned above in a section of xe2x80x9cRelated Artxe2x80x9d, the crystallization of the semiconductor film due to a metal element may be performed. In either one of these, optimum laser irradiation conditions are similar.
FIG. 3 is a graph showing relationship between a scanning speed of a semiconductor film and an optimum laser output for crystallizing the semiconductor film. A vertical axis shows the optimum laser output (unit: W), and a horizontal axis shows the scanning speed of the semiconductor film (unit: cm/s). In the present experiment, the maximum value of the scanning speed is 100 cm/s. From the graph, it is found that there is a linear relationship between the scanning speed and the output. For convenience of comparison with later experiments, from the graph shown in FIG. 3, it is estimated that when the laser output is 10 W, the optimum scanning speed of the semiconductor film is substantially 150 cm/s.
FIG. 4 shows an optical system in which a major axis of the elliptical beam is further elongated. Thereby, an elliptical beam 406 having a major axis of 700 xcexcm and a minor axis of 30 xcexcm can be formed. In the present specification, in order to standardizing experimental results, the minor axis of the elliptical beam is fixed at 30 xcexcm. A specific configuration of the optical system includes a laser oscillator 401, a mirror 402 that deflects a light path to a vertical direction, a cylindrical lens 403 that adjusts a length of the major axis of the elliptical beam and has a focal length of 150 mm, and a cylindrical lens 404 that adjusts a length of the minor axis and has a focal length of 20 mm. The cylindrical lens 403 is disposed 120 mm above the semiconductor film 405, and the cylindrical lens 404 is disposed so that a focal point thereof may converge on the semiconductor film 405. The cylindrical lenses 403 and 404 and the semiconductor film 405 are disposed perpendicularly to an optical axis of the laser beam.
When the elliptical beam 406 is scanned in a minor axis direction of the elliptical beam relative to the semiconductor film 405 and thereby the semiconductor film 405 is crystallized, a state where, in a region having a width of 250 xcexcm in the major axis direction of the elliptical beam, grains elongated in the scanning direction are closely packed can be formed. The optimum scanning speed at this time is 50 cm/s and the laser output is 10 W.
An optical system where the major axis of the elliptical beam is further elongated is shown in FIG. 5. Thereby, an elliptical beam 505 having a major axis of 2000 xcexcm and a minor axis of 30 xcexcm can be formed. A specific configuration of the optical system includes a laser oscillator 501, a mirror 502 that deflects a light path into a vertical direction, and a cylindrical lens 503 that adjusts a length of the minor axis of the elliptical beam and has a focal length of 20 mm. The cylindrical lens 503 is disposed so that a focal point may converge on the semiconductor film 504. The cylindrical lens 503 and the semiconductor film 504 are disposed so as to be at a right angle relative to the optical axis of the laser beam.
When the elliptical beam 505 is scanned in a minor axis direction of the elliptical beam relative to the semiconductor film 504 to crystallize the semiconductor film 504, a state where, in a region having a width in the range of 600 to 800 xcexcm in the major axis direction of the elliptical beam, grains elongated in the scanning direction are closely packed can be formed. The optimum scanning speed at this time is 5 to 10 cm/s and the laser output is 10 W.
A series of experimental results are shown in a graph of FIG. 6. Specifically, FIG. 6 shows the relationship between the optimum scanning speed of the crystallization of the semiconductor film and the width of the region of grains in the major axis direction of the formed elliptical beam when the laser output is fixed at 10 W and the width of the elliptical beam is set at 30 xcexcm. An irradiation target is the aforementioned a-Si film having a thickness of 150 nm. The vertical axis shows the scanning speed V (unit: cm/s) of the semiconductor film and the horizontal axis shows the width L (unit: xcexcm) of the long grain region. When both are shown on a full logarithmic plot, the relationship between these becomes substantially linear.
When the relationship between these is expressed with an equation,
log L=xe2x88x920.465 log V+3.188xe2x80x83xe2x80x83Equation 1)
can be obtained. In the present specification, for convenience of understanding, unit of the width L of the long grain region and that of the scanning speed V are made different. However, same unit may be employed, and in that case, only a constant term in the Equation 1) becomes different. Accordingly, except for the constant term, it can be said that the Equation 1) holds for any unit-systems.
On the other hand, when a time necessary for laser annealing is shown by T, T can be expressed as follow:
T=(a/L)xc3x97(b/V+2V/g)xe2x80x83xe2x80x83Equation 2)
(where a is a length (unit: xcexcm) of a shorter side when the semiconductor film is assumed to be rectangular, and b is a length (unit: cm) of a longer side when the semiconductor film is assumed to be rectangular, and g is acceleration (unit: cm/s2) necessary for the scanning speed to reach a velocity V). A factor 2 of the Equation 2) denotes an acceleration period and a deceleration period. Since the units of the a and L are unified, a/L is dimensionless. Accordingly, even in Equation 2), when all unit systems are unified, the same result can be obtained.
When an entire surface of a semiconductor film formed on a rectangular substrate is laser-annealed according to the present invention, it is obvious that when the elliptical beam is scanned along a longer side of a rectangular substrate the laser annealing can be most efficiently performed. At this time, a major axis of the elliptical beam is arranged so as to be in parallel with a shorter side of the substrate. When thus arranged, the number of times of the acceleration and deceleration of the scanning can be made the least. Accordingly, in Equation 2), it is assumed that a denotes a shorter side of a rectangle and b denotes a longer side thereof. However, since the crystal grain develops in a scanning direction of the laser beam, when in a rectangular substrate a semiconductor element in which grains elongated in the shorter side direction is necessary, there is no problem when a and b may replace each other.
Here, with reference to FIGS. 7A and 7B, the Equation 2) will be explained. In FIG. 7A, with an elliptical beam 7002, a semiconductor film 7001 is scanned in the direction shown with an arrow in the drawing. Since this movement is relative, either the elliptical beam 7002 or the semiconductor film 7001 may be moved or both may be moved. There is no essential difference among these. Since when the scanning speed of the semiconductor film 7001 is slow, a period necessary for the acceleration can be made substantially zero, g becomes infinity. However, since when the semiconductor film is scanned the semiconductor film is necessary to make reciprocating movement, as the scanning speed of the semiconductor film becomes larger, under an influence of the acceleration at both ends of the reciprocating movement, it becomes to take an excessive processing time T. That is, in addition to an irradiation time period (one shown by a region of a scanning distance b in FIG. 7B; the scanning speed at this time is constant), a longer acceleration time period (one shown by a region of a scanning distance c in FIG. 7B) is required. In the present specification, although g is treated as a constant, it is of course no problem even when the g is a function of time. In such case, g(t) can be time-averaged and thereby can be treated as a constant.
When logarithms of both sides of the Equation 2) are taken,
log T=log axe2x88x92log L+log(b/V+2V/g)xe2x80x83xe2x80x83Equation 3)
is obtained.
When L is eliminated from Equation 1) and Equation 3),
xe2x80x83log T=log(b/V0.535+2V1.465/g)+Axe2x80x83xe2x80x83Equation 4)
is obtained. Here, A is a constant (it is obvious from A=xe2x88x923.188+log a). In the above equation, the variable L depends on an output (10 W in this case), and when a minor axis of the elliptical beam is set at a constant, there is a substantial linear relationship. However, the variation of the output causes no difference in the meaning of the equation. When the output varies, only the constant term A of the equation varies. Accordingly, when a scanning speed V that minimizes the Equation 4) is obtained, it is obvious that whatever laser outputs may be, the obtained speed V makes a time necessary for the laser annealing the shortest. That is, the present invention provides the V that makes the Equation 4) smallest, or the V that can make the time necessary for laser annealing shortest.
When the Equation 4) is differentiated with respect to V,
(log T)xe2x80x2=f(V)(5.477/gxe2x88x92b/V2)xe2x80x83xe2x80x83Equation 5)
is obtained (here, f(V) is a function of V).
In order to carry out the laser annealing most efficiently, since the T need only take the least value, when the Equation 5) is equated to zero followed by calculation,
V=(gb/5.477)1/2xe2x80x83xe2x80x83Equation 6)
is obtained. That is, the laser annealing need only be carried out at the scanning speed V in conformity with the Equation 6).
A substrate used normally in the production line is a rectangular one having a size of, for instance, substantially 600 mmxc3x97720 mm. Accordingly, the longer side of the rectangle corresponds to b (=72). In an ordinary XY stage, since an acceleration is in the range of 10 to 1000 cm/s2, when the acceleration g is set at, for instance, 250 cm/s2, the V derived from the Equation 6) becomes 57 cm/s.
FIGS. 10A through 10C show relationship between the time period necessary for the laser annealing and the scanning speed of the semiconductor film. When the laser annealing is carried out at the V that is substantially one half to twice the scanning speed V calculated from the Equation 6), that is, in the range surrounded by solid lines in FIGS. 10A through 10C, the laser annealing can be efficiently carried out in a time that is substantially 1.3 times or less the time necessary for the shortest laser annealing. Accordingly, the laser annealing need only be carried out in the range of
(gb/5.477)1/2/2 less than V less than 2xc3x97(gb/5.477)1/2xe2x80x83xe2x80x83Equation 7).
Preferably, when the laser annealing is carried out at the V in the range of substantially 90% to 110% of the scanning speed calculated from Equation 6), that is, in the range surrounded by the dashed lines in FIGS. 10A through 10C, the laser annealing can be performed in a time substantially the same as that of the shortest annealing and as efficiently as that. Accordingly, the laser annealing may be carried out in the range of
0.9xc3x97(gb/5.477)1/2 less than V less than 1.1xc3x97(gb/5.477)1/2xe2x80x83xe2x80x83Equation 8).
Although the sizes of the substrates normally used are various such as 300xc3x97400 mm, 550xc3x97650 mm, (600 to 620 mm)xc3x97720 mm, 730xc3x97920 mm, 1000xc3x971200 mm and 1150xc3x971350 mm, whatever size of the substrate is used, the above calculation can be applied. Furthermore, although the calculation is conditioned on the elliptical beam, it is of course that the calculation can be applied to a shape close to this, such as, for instance, a rectangular shape or a bobbin-like shape. In order to get a rectangular beam, for instance, a slab type laser oscillator can be used. In order to obtain a bobbin-like beam, for instance, aberration of a lens can be used to form it.